1. Field of the Invention
The present invention relates to an improved circuit construction of a tuning circuit for improving frequency selectivity Q of the tuning circuit by using a negative resistance circuit.
2. Description of the Related Art
In a radio receiver, there are methods for improving Q of a tuning circuit by using a negative resistance circuit as methods for improving a frequency characteristic of a tuning circuit used therein. One of them is a method for compensating a series resistance component of a tuning circuit with said negative resistance circuit by constituting the tuning circuit so that a driving circuit is connected to a terminal of a series resonance circuit consisting of an inductor and a capacitor connected to the inductor in series to add an input signal thereto and a negative resistance circuit is connected to other terminal thereof to ground it. Said method requires that design and adjustment is easy and operation is stable. As a conventional method satisfying this requirement there is a method disclosed in Japanese Patent Application No. 2003-218036. This method uses a differential amplifying circuit having two transistors whose emitters are connected directly, a low impedance output circuit as a emitter follower, a positive feedback circuit for positively feeding back an output signal of said output circuit to a same phase input side of the differential amplifying circuit directly, and a negative feedback circuit for negatively feeding back said output signal to an inverse phase input side of the differential amplifying circuit through a negative resistance setting resistor to obtain a negative resistance between this inverse phase input terminal and earth. According to this method, since a negative resistance circuit can be constituted by only well known circuits, circuit design and adjusted thereof are easy. Further since it is a simple construction circuit, it operates stably to a comparative high frequency.
However, since the above circuits requires three transistors, as a circuit having more simple construction a tuning circuit (Japanese Patent Application No. 2002-257367) was proposed. This tuning circuit is constituted by two stage transistor amplifying circuits wherein a first stage circuit is a C-E dividing type (collector-emitter dividing) circuit and a second stage circuit is an emitter follower. A positive feedback circuit is constituted by connecting an output of the emitter follower to an emitter of the first stage circuit directly or though a resistor and a negative feedback circuit is constituted by negatively feeding back an output of the second stage emitter follower to a base input of the first stage circuit through a resistor. According this construction a negative resistor can be obtained between the base input and earth.
Since the above described circuit is constructed by a circuit having only two transistors, it has more simple construction than that of the three transistor circuit and its adjustment is easy.
However, since any stage of the two stage transistor circuit does not take high amplification basically, a negative resistance value thereof varies largely by fine change of amplification of a transistor owing to variation of temperature or power voltage, there is a problem that it is necessary to keep usable condition such as temperature or power voltage, etc. in very stable condition.